1. Field of the Invention
The present invention relates to a servo-control system, and more specifically to an integration-proportional controller in such a servo control system which controls a servomotor for example, satisfactorily in reference-value follow-up characteristics, and disturbance suppressing characteristics, etc.
2. Description of the Prior Arts
It is conventional in a servo-control system to employ an integration-proportional controller in driving any load such as a motor for removing load vibration and improving the responsibility of the load. The integration-proportional controller (hereinafter simply referred to as I-P controller) is to control the servomotor rapdily and smoothly in response to a speed reference-value by amplifying a deviation between the reference-value and a feedback signal.
Referring here to FIG. 6, such a conventional integration-proportional controller in a servo-control system is illustrated.
In the figure, a speed reference signal r is inputted into the I-P controller 2 and branched at a first control junction point 4 of summing and/or differencing to an integrator 6 and from a proportional amplifier 8. Outputs Ui and Up each from the integrator 6 and the proportional amplifier 8 are outputted at a second control junction point 10. An output from the point 10 is supplied to a motor 14 for driving a load (not shown) through a motor drive control unit 16 for driving the motor 14. The output from the control function point 10 is a difference between the respective outputs Ui and Up from the integrator 6 and the proportional amplfier 8, and is limited within preset upper and lower limits. The angle of rotation f of the motor 14 is detected by a pulse generator 18 which then converts the detected angle to a pulse number per revolution time and supplies it to a speed detector 20.
Referring further to FIG. 7, timing charts of the respective control signals in the conventional integration-proportional amplifier are illustrated.
In the figure, when the speed reference signal r is applied in a stepwise manner with the angle of rotation f of the motor 14 being zero, for example, as illustrated in FIG. 7(a), the aforementioned deviation .epsilon. (.epsilon.=r-f) is as illustrated in FIG. 7(b), which causes an increase of the output Ui from the integrator 3 as illustrated in FIG. 7(c).
The output Ui from the integrator 6 increases without interruption until the rotation angle of the motor 14 is equal to the speed reference signal r, allowing the deviation to be negative thereafter, followed by the output Ui of the integrator 3 decreasing, after a while to a certain constant value.
Here, the conventional I-P compensator 2 has a drawback as follows.
With the speed reference signal r being larger or with the inertia of the motor 14 being greater, or with the upper limit of the motor drive control unit 16 being low, the I-P controller 2 has much time to return in its original state. In such a situation where the output Ui from the integrator 3, which has received the large deviation .epsilon. and kept its integration operation over a long period of time, becomes a very large value, suffering from taking much time to return to the normal value even if the motor 14 reaches the speed reference signal r and the deviation .epsilon. is reversed in its polaritY. This delays the return of the I-P controller 2 to its original state, thus further delaying the response of the control.
Here, another situation of the conventional I-P controller will be described with reference to FIGS. 8 through 10. When a servomotor is coupled with a mechanical unit to drive the latter, a stepwise speed reference signal is applied to the I-P controller as described previously. The mechanical output of the servomotor increases rapidly to transmit the torque to the mechanical unit very rapidly. This causes mechanical vibration on the mechanical unit because of its gear backlash, deformation of a shaft, and deflection of a belt, etc. To solve such a problem, an S-shaped instruction signal generator is conventionally available which converts the stepwise instruction signal to a smoothly changing instruction signal.
Referring here to FIG. 8 such S-shaped speed reference signal generator 21 is illustrated.
In the figure, a speed reference signal REF is inputted into the S-shaped speed reference signal generator 21 at a junction point 22 where it is subtracted by a feedback signal fb into a deviation signal .epsilon.. The deviation signal is fed to a logical discriminator 23 which issues a Up or ZERO signal at its output terminal UP or ZERO depending upon a feedback signal ACC issued from a first integrator 24 after receiving the UP or ZERO signal from the logic discriminator 23. An output ACC from the first integrator 24, which partly forms the aforementioned feedback signal ACC, is fed to a second integrator 25. The second integrator 25 issues the aforementioned feedback signal fb to the junction point 22 on one side and an instruction signal WR to a servomotor control unit (not shown but described later) on the other hand. The instruction signal WR is to control the servomotor control unit.
Referring to FIG. 9, output waveforms at respective portions of the S.TM.shaped speed reference signal generator 21 are illustrated for description of the operation of the generator 21.
In the figure, with the speed reference signal REF being inputted to the generator 21 at the branch point 22 to cause the deviation signal .epsilon. to rise with the same amplitude as that of the speed reference signal REF, the terminal Up of the logical discrimiator 23 is activated to permit the ouput ACC from the first integrator 24 to rise in a ramp waveform at time t0. When the output reaches a predetermined value B at time t1, the terminal Up of the logical discriminator 23 is inactivated to keep thereafter the ouput ACC at the predetermined value B. The speed reference signal WR from the second integrator 25 is increased during a time interval t0 to t1 drawing substantially a quadratic curve. When the deviation signal .epsilon. is decreased to satisfy a relation .epsilon..ltoreq.A (&lt;B), the terminal ZERO of the logical discriminator 23 is activated, causing the ouput ACC to be decreased toward zero in a ramp waveform and hence the speed reference singal WR to be settled to the speed reference signal REF.
Referring in succession to FIG. 10, a positional servo-control system including the foregoing S-shaped speed reference signal generator is illustrated.
In the figure, an error amplifier 27 after receiving a position command signal 28 through a junction point 29 amplifies and ouputs it to an S-shaped speed reference signal generator 21. The S-shaped speed reference signal generator 21 outputs a smooth speed reference signal WR2 converted therein from the stepwise speed reference signal REF to a servomotor control unit 31, which corresponds to the motor drive control unit 16 of FIG. 6. The servomotor control unit 31 hereby controls a motor 32, the rotation of which motor is transmitted to a pulse generator 33 and to a load 34. The pulse generator 33 feeds back a feedback signal 36 to the junction point 29 and to a speed detector 35.
Hereupon, the S-shaped speed reference signal generator 21 described above has a drawback that the generator, which might often be incorporated into the control loop as described above referring to FIG. 10, has basically an integration characterisitc. This causes the control system including the generator in its control loop to be unstable